The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP 3rd generation partnership project
ACK acknowledgement
BS base station
BTS base transceiver station
CN core network
DL downlink
DPCCH dedicated physical control channel
E-DCH enhanced data channel
E-UTRA evolved universal terrestrial radio access
eNB, eNodeB evolved node B/base station in an E-UTRAN system
E-UTRAN evolved UTRAN (LTE)
DTX discontinuous transmission
DRX discontinuous reception
HNB home node B
HPN high power node BS
HS-DPCCH high speed dedicated physical control channel
HUE UE served by HPN
HSPA high speed packet access
ICIC inter-cell interference coordination
LPN low power node BS
LUE UE served by LPN
LTE long term evolution
LTE-A long term evolution advanced
ICIC inter-cell interference coordination
NACK negative acknowledgement
NB Node B
O&M operations and maintenance
RACH random access channel
RAT radio access technology
RGCH relative grant channel
RNC radio network controller
ROT rise over thermal noise
Rx, RX reception, receiver
SHO soft handoff
TDM time division multiplexing
Tx, TX transmission, transmitter
UE user equipment
UL uplink
UPH UE power headroom
Recently heterogeneous deployments where low power nodes are placed throughout a macro-cell layout have gained significant interest from cellular network operators as a means to enhance system performance (coverage and capacity). The co-channel heterogeneous network deployment in which small power nodes use the same carrier frequency as the macro-cell is presented for study of HSPA evolution in 3GPP in R1-110687, “Interference Issues in Heterogeneous Networks for HSPA”, QUALCOMM Incorporated, Taipei, Taiwan, 21-25 Feb. 2011. Under co-channel deployment, introduction of the low power nodes to the macro-cell brings challenges in terms of the control channel (such as HS-DPCCH) reliability as well as the interference management between low and high power nodes.
From the high level architecture point of view, one scenario for the deployment of small power nodes could be a dedicated controller deployment where the macro-cells and the low power nodes are controlled by different RNCs. The dedicated controller deployment allows operators more flexible choice of vendors, and could be a valuable solution in case if the RNC runs into the issue of capacity limit and/or port connectivity limit (e.g., in the NodeB). Under this scenario, SHO will not be supported between macro-cell and small power nodes. On the positive side, there will be no control channel (such as HS-DPCCH) reliability problem. However, on the negative side, interference problems could become more severe.
The interference problems comes from the transmit power difference between low power nodes (LPNs) such as femto cell BS, and the high power nodes (HPNs) such as macro cell BS. As serving cell selection and active set management are mainly based on the DL received signal strength, transmit power of each cell largely determines the coverage area of the cell. Normally, a high transmit power nodes will cover larger area than the low transmit power nodes. However, from an UL perspective, the strength of the signal being received at each node does not rely on the DL transmit power of each node. Consequently, introduction of the low power nodes could potentially cause large UL imbalance, i.e., UL cells other than the serving cell could receive much stronger signals from the UE than the serving cell.
In the deployment of heterogeneous networks, there is potentially one problem that could arise from the introduction of the LPNs, i.e., excessive UL interference from HPN UEs (HUEs) to the low power nodes.
The excessive interference in victim LPNs is caused by the UEs being served by the HPN while not having the victim LPNs in the active set. In this case, due to the UL imbalance, the UE could still have a better UL to the small power node than to the serving cell. Not being in the active set, the SPNs could not power control the UE and/or limit the UE grant by a RGCH. Consequently, these LPNs could be the victims of a large un-controllable interference from HPNs. As a result, UEs served by the victim LPNs may suffer from bad UL throughput.
The relation between DL received powers at a UE side from the HPN and LPN can be expressed as follows:Ptx—dl—h−PL—hu=Ptx—dl—l−PL—lu+Offset  (1),
where Ptx_dl_h is a DL transmission power from the HPN, PL_hu is a pathloss between the HPN and the UE, Ptx_dl_l is a downlink transmission power from the LPN, PL_lu is a pathloss between the LPN and the UE. Equation 1 may be further presented asPL—hu−Pl—lu=Ptx—dl—h−Ptx—dl—l−Offset  (2).
The relation between UL received powers at the HPN and LPN can be expressed as follows:Prx—ul—l=Prx—ul—h+(Ptx—dl—h−Ptx—dl—l)−Offset  (3),
where Pr x_ul_h is a UL received power by the HPN and Pr x_ul_l is a UL received power by the LPN. Equation 3 may be further presented asPrx—ul—l=Prx—ul—h+PowerDiff−Offset  (4).
Moreover, one simple example is shown in FIG. 1 which illustrates the UL interference from the UE 16 in HPN cell to the LPN 12.
In the example of FIG. 1, the Tx power at the serving HPN 14 is assumed to be 43 dBm and the Tx power at a neighbor LPN 12 is assumed to be 33 dBm. Then the received uplink interference power at the LPN 12 from the UE 16 without SHO support can be expressed as:Prx—ul—l=Prx—ul—h+10 dB−Offset  (5),
where Offset indicates the received power difference between the HPN 14 and LPN 12 in the DL, which is also related to the setting of a serving cell change parameter. In other words, for larger Offset values, the UE 16 is closer to the HPN 14 and the UL interference to the LPN 12 is smaller according to Equation 5. Otherwise, the uplink interference would be larger.
Some documents provide discussions related to the problem of interference described herein. For example, the conclusion from the document 3GPP TR 25.820 v8.2.0, “3G Home NodeB Study Item Technical Report”, 09/2008, states the following: “The femtocell receiver must reach a compromise between protecting itself against uncoordinated interference from the macro UEs, while controlling the interference caused by its own UEs towards the macro layer. Adaptive uplink attenuation can improve performance but consideration must also be given to other system issues like the associated reduction in UE battery life.”
Furthermore, some solutions mentioned in “Interference Management in UMTS Femtocells”, www.femtoforum.org, Femto Forum, 12/2008, are summarized as follows:
1. Availability of alternative resources (a second carrier, or underlay RAT) for handing off or reselecting macro users is the best way to provide good service in the case where macro users are in the proximity of femtocells.
This approach implicitly relies on the escape carrier to avoid the co-channel interference which will require the multiple carriers or RATs as a tradeoff
2. The femtocell (a kind of LPN) is required to cope with UL interference from UE being served by the macrocell. An increase of 20 dB in the minimum requirements of dynamic range performance was proposed and subsequently incorporated as a performance measure to be applied, see 3GPP TS 25.104 v10.3.0, “Base Station (BS) radio transmission and reception (FDD)”, 09/2011.
This approach may work for the case of only a few femto UEs. However, in case of multiple femto UEs, the unstable ROT caused by the dramatically changed interference from the aggressor UEs (served by and in communication with the HPN/BS)) will cause inefficient uplink scheduling at the BS.